Many reports have appeared which are directed to the targeting of tumor cells with monoclonal antibody-drug conjugates {Sela et al, in Immunoconjugates, pp. 189–216 (C. Vogel, ed. 1987); Ghose et al, in Targeted Drugs, pp. 1–22 (E. Goldberg, ed. 1983); Diener et al, in Antibody Mediated Delivery Systems, pp. 1–23 (J. Rodwell, ed. 1988); Pietersz et al, in Antibody Mediated Delivery Systems, pp. 25–53 (J. Rodwell, ed. 1988); Bumol et al, in Antibody Mediated Delivery Systems, pp. 55–79 (J. Rodwell, ed. 1988); G. A. Pietersz & K. Krauer, 2 J. Drug Targeting, 183–215 (1994); R. V. J. Chari, 31 Adv. Drug Delivery Revs., 89–104 (1998); W. A. Blattler & R. V. J. Chari, in Anticancer Agents, Frontiers in Cancer Chemotherapy, 317–338, ACS Symposium Series 796; and I. Ojima et al eds, American Chemical Society 2001}. Cytotoxic drugs such as methotrexate, daunorubicin, doxorubicin, vincristine, vinblastine, melphalan, mitomycin C, chlorambucil, calicheamicin and maytansinoids have been conjugated to a variety of murine monoclonal antibodies. In some cases, the drug molecules were linked to the antibody molecules through an intermediary carrier molecule such as serum albumin {Garnett et al, 46 Cancer Res. 2407–2412 (1986); Ohkawa et al, 23 Cancer Immunol. Immunother. 81–86 (1986); Endo et al, 47 Cancer Res. 1076–1080 (1980)}, dextran {Hurwitz et al, 2 Appl. Biochem. 25–35 (1980); Manabi et al, 34 Biochem. Pharmacol. 289–291 (1985); Dillman et al, 46 Cancer Res. 48864891 (1986); and Shoval et al, 85 Proc. Natl. Acad. Sci. U.S.A. 8276–8280 (1988)}, or polyglutamic acid {Tsukada et al, 73 J. Natl. Canc. Inst. 721–729 (1984); Kato et al, 27 J. Med. Chem. 1602–1607 (1984); Tsukada et al, 52 Br. J. Cancer 111–116 (1985)}.
A wide array of linkers is now available for the preparation of such immunoconjugates, including both cleavable and non-cleavable linkers. In vitro cytotoxicity tests, however, have revealed that antibody-drug conjugates rarely achieve the same cytotoxic potency as the free unconjugated drugs. This has suggested that mechanisms by which drug molecules are released from conjugated antibodies are very inefficient. Early work in the area of immunotoxins showed that conjugates formed via disulfide bridges between monoclonal antibodies and catalytically active protein toxins were more cytotoxic than conjugates containing other linkers {Lambert et al, 260 J. Biol. Chem. 12035–12041 (1985); Lambert et al, in Immunotoxins 175–209 (A. Frankel, ed. 1988); Ghetie et al, 48 Cancer Res. 2610–2617 (1988)}. This improved cytotoxicity was attributed to the high intracellular concentration of reduced glutathione contributing to the efficient cleavage of the disulfide bond between the antibody molecule and the toxin. Maytansinoids and calicheamicin were the first examples of highly cytotoxic drugs that had been linked to monoclonal antibodies via disulfide bonds. Antibody conjugates of these drugs have been shown to possess high potency in vitro and exceptional antitumor activity in human tumor xenograft models in mice {R. V. J. Chari et al., 52 Cancer Res., 127–131 (1992); C. Liu et al., 93, Proc. Natl. Acad. Sci., 8618–8623 (1996); L. M. Hinman et al., 53, Cancer Res., 3536–3542 (1993); and P. R. Hamann et al, 13, BioConjugate Chem., 4046 (2002)}.
An attractive candidate for the preparation of such cytotoxic conjugates is CC-1065, which is a potent anti-tumor antibiotic isolated from the culture broth of Streptomyces zelensis. CC-1065 is about 1000-fold more potent in vitro than are commonly used anti-cancer drugs, such as doxorubicin, methotrexate and vincristine {B. K. Bhuyan et al., Cancer Res., 42, 3532–3537 (1982)}.
The structure of CC-1065 (Compound 1, FIG. 1A) has been determined by x-ray crystallography {Martin, D. G. et al, 33 J. Antibiotics 902–903 (1980), and Chidester, C. G., et al, 103 J. Am. Chem. Soc. 7629–7635 (1981)}. The CC-1065 molecule consists of 3 substituted pyrroloindole moieties linked by amide bonds. The “A” subunit has a cyclopropyl ring containing the only asymmetric carbons in the molecule. While only the relative configuration of these carbons is available from x-ray data, the absolute configuration has been inferred as 3b-R, 4a-S, by using DNA as a chiral reagent {Hurley, L. H. et al, 226 Science 843–844 (1984)}. The “B” and “C” subunits of CC-1065 are identical pyrroloindole moieties.
The cytotoxic potency of CC-1065 has been correlated with its alkylating activity and its DNA-binding or DNA-intercalating activity. These two activities reside in separate parts of the molecule. Thus, the alkylating activity is contained in the cyclopropapyrroloindole (CPI) subunit and the DNA-binding activity resides in the two pyrroloindole subunits (FIG. 1A).
However, although CC-1065 has certain attractive features as a cytotoxic agent, it has limitations in therapeutic use. Administration of CC-1065 to mice caused a delayed hepatotoxicity leading to mortality on day 50 after a single intravenous dose of 12.5 μg/kg {V. L. Reynolds et al., J. Antibiotics, XXIX, 319–334 (1986)}. This has spurred efforts to develop analogs that do not cause delayed toxicity, and the synthesis of simpler analogs modeled on CC-1065 has been described {M. A. Warpehoski et al., J. Med. Chem., 31, 590–603 (1988)}. In another series of analogs, the CPI moiety was replaced by a cyclopropabenzindole (CBI) moiety {D. L. Boger et al., J. Org. Chem., 55, 5823–5833, (1990), D. L. Boger et al., BioOrg. Med. Chem. Lett., 1, 115–120 (1991)}. These compounds maintain the high in vitro potency of the parental drug, without causing delayed toxicity in mice. Like CC-1065, these compounds are alkylating agents that bind to the minor groove of DNA in a covalent manner to cause cell death. However, clinical evaluation of the most promising analogs, Adozelesin and Carzelesin, has led to disappointing results {B. F. Foster et al., Investigational New Drugs, 13, 321–326 (1996); I. Wolff et al., Clin. Cancer Res., 2,1717–1723 (1996)}. These drugs display poor therapeutic effects because of their high systemic toxicity.
The therapeutic efficacy of CC-1065 analogs can be greatly improved by changing the in vivo distribution through targeted delivery to the tumor site, resulting in lower toxicity to non-targeted tissues, and thus, lower systemic toxicity. In order to achieve this goal, conjugates of analogs and derivatives of CC-1065 with cell-binding agents that specifically target tumor cells have been described {U.S. Pat. Nos. 5,475,092; 5,585,499; 5,846,545}. These conjugates typically display high target-specific cytotoxicity in vitro, and exceptional anti-tumor activity in human tumor xenograft models in mice {R. V. J. Chari et al., Cancer Res., 55, 4079–4084 (1995)}.
Cell-binding agents are typically only soluble in aqueous medium, and are usually stored in aqueous solutions. Thus, these analogs should possess sufficient water solubility to allow for efficient reaction with cell-binding agents and subsequent formulation in aqueous solution. In addition, for cell-binding agent conjugates to have a useful shelf life, it is important that CC-1065 analogs that are linked to these cell-binding agents are stable for an extended period of time in aqueous solutions.
The CC-1065 analogs described thus far (see, e.g. FIGS. 1B and 1C) are only sparingly soluble in water. Because of the sparing solubility of CC-1065 analogs, conjugation reactions with cell-binding agents currently have to be performed in extremely dilute aqueous solutions. Therefore, these prodrugs should have enhanced water solubility as compared to the parent drugs.
Also, CC-1065 analogs that have been described thus far are quite unstable in aqueous solutions for the following reason. The seco-form of the drug is spontaneously converted into the cyclopropyl form, which then may alkylate DNA, if present. However, the competing reaction of the cyclopropyl form with water results in opening of the cyclopropyl ring to yield the hydroxy compound, which is inactive. Thus, there is a need to protect the reactive portion of CC-1065 analogs in order to extend their useful life in aqueous solution, for example by the development of prodrugs of CC-1065 analogs.
There is therefore a need to develop prodrugs of CC-1065 analogs that are very stable upon storage in aqueous solutions. Preferably, these prodrugs should only be converted into active drugs in vivo. Once the prodrug is infused into a patient, it should preferably be efficiently converted into active drug.
Carzelesin is a prodrug where the phenolic group in adozelesin is protected as a phenyl carbamate {L. H. Li et al., Cancer Res., 52, 4904–4913 (1992)}. However, this prodrug is too labile for therapeutic use, and also affords no increase in water solubility compared to the parental drug. In a second example, the phenolic residue of a CC-1065 analog was glycosylated to produce a prodrug (U.S. Pat. No. 5,646,298). However, this prodrug is not converted into active drug in vivo, and requires the additional administration of an enzyme from a bacterial source to convert it to the cytotoxic form.
There are a few examples of anticancer drugs, unrelated to CC-1065, that have been converted into water soluble prodrugs. In the anticancer drug irinotecan, the phenolic group is protected by a 4-piperidino-piperidino carbamate. It has been reported that this protecting group confers water solubility to the drug. In addition, the prodrug is readily converted in vivo in humans to the active drug, presumably by the enzyme carboxylesterase, which naturally exists in human serum, tumor tissue and in some organs {A. Sparreboom, 4, Clin. Cancer Res., 2747–2754 (1998). L. P. Rivory et al., 52, Biochem Pharmacol., 1103–1111 (1996)}.
Similarly, the anticancer drug etoposide phosphate is an example of a prodrug that has a phosphate protecting group and is rapidly converted into active drug in vivo, presumably through hydrolysis by endogenous alkaline phosphatase {S. Z. Fields et al., 1 Clin. Cancer Res., 105–111 (1995)}.
Thus, there exists a need for analogs of CC-1065 that have increased solubility and stability in aqueous solution, to facilitate their conjugation to cell binding agents in aqueous solutions, while preserving their biological activity. In addition, in order to reduce toxic side-effects, it would be advantageous to provide the CC-1065 analog in the form of a prodrug that is converted to the cytotoxic drug predominantly at the desired therapeutic site and preferably through the action of endogenous agents. All these advantages and more are provided by the invention described herein, as will be apparent to one of skill in the art upon reading the following disclosure and examples.